In a broadband wireless communication system, a Base Station (BS) is in charge of a geographical region referred to as a cell that is determined according to a radio-wave propagation distance. That is, the BS communicates with Mobile Stations (MSs) existing within a cell of the BS through a wireless channel. In this case, mutual interference exists between neighboring cells. That is, in case of downlink communication, an MS located in a cell boundary experiences interference due to a signal transmitted from a BS of a neighboring cell, and in case of uplink communication, the BS experiences interference due to a signal transmitted from the MS located in the neighboring cell boundary.
To cancel interference from the neighboring cell, that is, to remove signals other than a Transmit (Tx)/Receive (Rx) signal of a target MS, the BS performs beamforming on the Tx/Rx signal. The beamforming is an operation for transforming the Tx/Rx signal by using a beamforming vector. A format of the beamforming vector varies depending on the purpose of beamforming. Therefore, if the purpose of beamforming is minimization of interference from the neighboring cell, a top priority is to decrease a size of an interfering signal when the beamforming vector is determined.
In case of using the beamforming vector whose top priority is to decrease the size of the interfering signal, if a target channel matrix and an interfering channel matrix are independent from each other, the BS may smoothly transmit and receive a target signal. However, if independence is not fully maintained between the target channel matrix and the interfering channel matrix, beamforming for interference cancellation has an effect on the target signal. For example, a beam pattern may change as illustrated in FIGS. 1A to 1C according to a correlation between the target channel matrix and the interfering channel matrix. As illustrated in FIGS. 1A to 1C, the closer the direction of the interfering signal to the direction of the target signal, the smaller the size of the target signal.
As described above, when beamforming for interference cancellation is performed, the interference cancellation results in degradation of the target signal, which leads to an adverse affect to link quality with the target MS. That is, even if interference is cancelled, significant decrease in the size of the target signal results in deterioration of communication quality. Accordingly, there is a need for a method for solving a problem in which the size of the target signal significantly decreases when beamforming is performed to improve communication quality by interference cancellation.